My battery went dead on me a couple of times lately. My alternator light comes on at idle and doesn't go away until I rev it up quite a bit. I checked the voltage on the battery and it goes up to about 13.5 volts while I'm reving the engine so it looks like I've got a charging problem only at idle.

Here's my question: Do I have to replace the alternator or may it just be the voltage regulator? How can I check which one it is?

Thanks,

Scott

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Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

The easy/cheap way is to try replacing the voltage regulator first. That's the most likely problem, but if it doesn't fix it you will have to look at the alternator and you will still have a spare voltage regulator.

last weekend. Driving home one night last week I progressively lost all electrical power. It was just like Apollo 13. Fortunately, the diesel is completely mechanical so even though I was running off the battery and depleted it, the motor continued to run with no problem.

Turns out the voltage regulator that bolts onto the back of the alternator and houses the brushes, needed to be replaced due to completely worn out brushes. Ten minute job and $34 for the part. I would pull the regulator out and inspect the brushes.

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2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

I just pulled the regulator. The brushes are shot. I hope a new one will take care of the problem. Don't want to have to spend $150 on a new alternator.

Scott

__________________
Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

Sounds like you have the problem identified. One other possibility would have been a loose drive belt.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970